Field of Invention
The invention relates in general to an improved pollution control device to destroy volatile organic compounds ("VOCs") contained in the exhaust or effluent of industrial and commercial processes. More particularly, it relates to a unit having a bed that includes both adsorbent and catalytic material that permits removal of VOCs from an effluent while simultaneously catalytically converting the VOCs into carbon dioxide and water.
Related Art
Systems to treat VOCs contained in the effluent of industrial processes are known. Conventional VOCs control systems typically utilize combustion processes for the conversion of VOCs into non-noxious compounds, such as carbon dioxide and water, as well as noxious compounds, such as nitrogen oxides and inorganic acids. Traditional combustion processes include: (1) thermal destruction of the VOCs; (2) thermal destruction of the VOCs with heat recovery; (3) catalytic thermal destruction of the VOCs and (4) a three-step process where first VOCs are removed from a waste stream and concentrated using an adsorbent, then the adsorbent is regenerated using a smaller quantity of air or other gas which creates a concentrated process stream, and finally the VOCs are destroyed in an additional processing unit. The thermal requirement and complexity of equipment needed for these various processes relative to basic thermal destruction are shown in Table 1 below. This table illustrates that improved thermal efficiency is typically correlated with increased process complexity for conventionally known systems. Hence, energy savings are achieved by employing complicated operations which have increased capital costs.
TABLE 1 ______________________________________ Relative Relative Cost Thermal & Complexity Requirement (Range 1-5) ______________________________________ 1. Thermal Destruction 1 1 2. Thermal Destruction with Heat 0.65 3 Recovery 3. Catalytic Thermal Destruction 0.4 2 4. Concentration Followed by Thermal Destruction Using Two Separate 0.2 5 Process Units ______________________________________
Accordingly, an effective system is needed to destroy VOCs having both a relatively low cost-complexity factor and a relatively low thermal requirement. A review of related VOC treatment apparatus below shows that conventional pollution control devices do not satisfy this need.
One method of treating exhaust gas (in the automotive environment) as it increases in temperature from a low temperature (ambient) to normal operating temperature (&gt;300.degree. C.) is disclosed in U.S. Pat. No. 4,985,210 to Minami and U.S. Pat. No. 5,140,811 to Minami, et al. In this method, VOCs are initially collected in an adsorbent trapper when the automobile exhaust gas is at a low temperature. When the automobile exhaust gas reaches a temperature greater than 300.degree. C., the VOCs from the automobile, as well as the VOCs desorbed from the adsorbent trapper, are catalytically converted into purified hydrocarbons in a second unit. Aside from being directed to the specialized field of automotive exhaust, this method suffers from several disadvantages, including higher thermal requirements and the necessity of two separate processing units.
Similar treatment systems are discussed in the patents to Patil et al., U.S. Pat. No. 5,125,231, and Dunne, U.S. Pat. Nos. 5,078,979 and 5,142,864, that serially treat automotive exhaust gases from a cold, start-up temperature to a normal operational temperature. Two phases are discussed: in the first phase, the gas temperature is lower than that which is necessary to achieve proper conversion in the catalytic reactor; and in the second stage, the gas temperature is such that proper catalytic conversion is possible. Patil ('231) discloses a catalytic converter that includes molecular sieves and a catalyst, yet disadvantageously requires a second catalytic converter. Therefore, these patents suffer from the same problems discussed above.
There are also patents that disclose continuous regeneration of adsorbent material. For example, U.S. Pat. No. 4,778,492 to Dawson discloses a rotatable drum having adsorbent material for adsorbing particular components of a gas stream. As the drum rotates, a portion of the filter media is passed through a liquid regenerating material that desorbs these particular collected components from the adsorbent material into a liquid flow stream. Another gas treatment device is shown in U.S. Pat. No. 3,183,649 to Teller, which discloses an apparatus for the separation of solvent vapors from noxious gases using a rotatable filter that is thermally desorbed using steam directed to one portion of the media.
The foregoing demonstrates that there is a need for an energy-efficient VOCs removal and destruction system that minimizes capital costs and process complexity, regenerates the adsorbent material without creating additional waste streams, and provides for an essentially uninterrupted operation.